This invention relates to a flexible scintillator for use with a nuclear (radiation-type) level detector. More particularly, the invention relates to a flexible scintillator made of an elongated stack of relatively thin layers of plastic scintillator material which may be operably connected at an end to a photodetector.
It is well known to use the combination of a radiation source, such as Cesium137 and an elongated radiation detector as a device for measuring the level of material, such as in a tank, that is situated between the radiation source and radiation detector. Such devices are particularly useful when the material being measured is particularly caustic, dangerous, or otherwise not amenable to traditional level measurement devices.
Early continuous level detection devices used an ion chamber detector. For example, the ion chamber could be a four to six inch diameter tube up to 20 feet long filled with inert gas pressurized to several atmospheres. A small bias voltage is applied to a large electrode inserted down the center of the ion chamber. As gamma energy strikes the chamber, a very small signal (measured in picoamperes) is detected as the inert gas is ionized. This current, which is proportional to the amount of gamma radiation received by the detector, is amplified and transmitted as the level measurement signal.
Alternatively, elongated scintillation detector xe2x80x9ccrystalsxe2x80x9d have been used. Such devices are many times more sensitive than ion chambers and are also considerably more expensive. This added expense is often acceptable because it allows the use of either a smaller radiation source size or to obtain a more sensitive gauge. When gamma energy hits the scintillator material, it is converted into visible or UV flashes comprised of light photons (particles of light). These photons increase in number as the intensity of gamma radiation increases. The photons travel through the scintillator medium to a photomultiplier tube, which converts the light photons into an electrical signal. The output is directly proportional to the gamma energy that is striking the scintillator.
Both ion chamber detectors and scintillation counter detectors have the disadvantage of being quite rigid in structure. In some applications, such as extending the detector vertically along the length of a tank, the shape of the tank or obstructions which are on or part of the tank limit or prevent the use of such detectors. There is a need for a scintillation counter detector that is flexible so that it may be adapted in the field to bend around such obstacles.
Fiber optic cables made of many individually clad strands of scintillator material have been presented as a solution to this problem. The required individual cladding of these fibers, however, makes such a solution undesirably costly.
The present invention provides a flexible scintillator, or flexible radiation-type level detector, in which a plurality of elongated, relatively thin layers of plastic scintillator material are stacked in close proximity to one another in a slidable relationship. The stack will have first and second ends, at least one of which is aligned for operable connection to a photodetector. Such a scintillator is flexible in at least two dimensions.
According to more particular aspects of the invention, the plastic scintillator material may include polyvinyltoluene and an anti-friction material, such as a film of polytetrafluoroethylene, may be added between layers of the plastic scintillator material.
Also in preferred form, edge and outer face surfaces of the layers of scintillator material may be covered with an inwardly-facing light reflective material and/or a light-excluding material.
Other aspects and features of the present invention will be noted upon examination of the drawings, description of the best mode for carrying out the invention, and claims, all of which constitute disclosure of the present invention.